Fwd: Re: Species Concept Question

[Steve Manning]

>Date: Tue, 25 May 2004 19:11:02 -0500
>To: Richard Pyle <deepreef at BISHOPMUSEUM.ORG>
>From: Steve Manning <sdmanning at asub.edu>
>Subject: Re: Species Concept Question
>
>At 11:08 AM 5/25/2004 -1000, you wrote:
>>Hello all,
>>
>>I have a sneaking suspicion that this is an incredibly naïve question,
>>because it seems to me to be so basic that it should have an obvious answer.
>>
>>Consider the tropical oceans, and the pattern of coral-reef habitat
>>throughout the ocean.  Unlike in some terrestrial environment, there are
>>clear "units" of habitat (islands & island groups) separated by vast regions
>>of non-habitat (open ocean).  It's a patch environment on a (literally)
>>global scale.
>>
>>Now, consider the following two patterns of distribution.  "X" refers to one
>>morphotype, and "O" refers to what appears to be a sister morphotype. Both
>>morphotypes are not subtly different from each other -- that is, they are
>>dramatically different enough from each other, that even the most ardent
>>"lumper" would be inclined to recognize them as distinct at the species
>>level. The lowercase "x" and "o" symbols in the patterns below refer to
>>individuals that are intermediate between the two morphotypes (i.e.,
>>hybrids).  The clusters represent island groups across a vast swath of ocean
>>(A, B, C, D, E).
>>
>>Pattern 1:
>>   A       B       C       D       E
>>-----------------------------
>>XXX      XXX    Xox     OOO     OOOO
>>  XXX    XXXX     OxO     OOO    OOO
>>XXXX     XX     xoXO    OOOO    OOOO
>>
>>Pattern 2:
>>   A       B       C       D       E
>>-----------------------------
>>XxX      XXo    Xox     OoX     OOOO
>>  XXX    XOxX     OxO     OOx    OOx
>>XoXX     XX     xoXO    OoOO    OoOO
>>
>>In both patterns, there is clear evidence of gene flow between the two
>>morphotypes in areas of sympatry (i.e., functionally no reproductive
>>barriers).  In the case of Pattern 2, there is a reasonably steady cline in
>>relative abundance of one form to the other, with relatively smooth
>>transitions of intermediates as well.  This is a pattern that taxonomists
>>occasionally have to wrestle with, which brings up all sorts of
>>philosophical species-concept questions.  That's not the pattern I'm
>>interested in right now.
>>
>>The first pattern represents a case where there are two clearly two distinct
>>morphotypes -- as I said, so distinct that even the most dedicated lumper
>>would treat them as separate species.  The two morphotypes are clearly each
>>other's closest relative, and have a generally parapatric distribution with
>>respect to each other. Throughout most of the ranges of each of the
>>morphotypes, there is tremendous consistency of form (i.e., very little
>>geographic variation within each of the respective ranges); except for the
>>zone of sympatry.  In that zone, you find a complete spectrum of
>>individuals, ranging from essentially pure "X" to pure "O", with every
>>imaginable intermediate in-between.  A classic "hybrid swarm".
>>
>>My question is difficult to articulate (I understand it in my head, but am
>>having trouble finding the right words to communicate it). But I'll give it
>>a shot:
>>
>>When confronted with Pattern 1, which of the three nomenclatural solutions
>>do you feel is the one that best facilitates communication among biologists:
>>
>>- One species epithet, noting two distinct geographic variants.
>>- Two species epithets, nothing a zone of hybridization where sympatric.
>>- One species epithet and two subspecies epithets, noting a zone of
>>hybridization where sympatric.
>>
>>???
>
>I prefer the second of the three choices.
>
>
>
>
>
>>But this is only part of my question.  A deeper level of the question has to
>>do with the role of distribution patterns and historical nomenclature in
>>making nomenclatural decisions.  For example, if in Pattern 1 population "C"
>>did not exist, would you be more inclined to treat them as separate species?
>>The gut-reaction answer is "Yes, because without evidence of gene flow
>>between two clearly distinct forms, one is to assume that they are distinct
>>species".  But if population "C" didn't exist, how would we ever know
>>whether the two forms were capable of freely interbreeding?  If, on the
>>other hand, the pattern of distribution looked like this:
>>
>>   A       B       C       D       E
>>-----------------------------
>>XXX      XXX    XOX     OOO     OOOO
>>  XXX    XXXX     OXO     OOO    OOO
>>XXXX     XX     XOXO    OOOO    OOOO
>>
>>That is, no evidence of gene flow between the two populations in an area of
>>sympatry, then most of us would have no trouble arriving at the conclusion
>>that these are distinct species (i.e., given the opportunity to interbreed,
>>they do not).
>>
>>But if, as I posited above, population "C" doesn't even exist (to our
>>knowledge), how do we assess whether the two morphotypes should be regarded
>>as distinct species (old question, I know -- and mostly rhetorical.  Also,
>>not really at the heart of what I'm after -- as I said, I can't quite figure
>>out how to articulate the real question I'm after).
>>
>>Now, suppose throughout most of the relevant nomenclatural history,
>>population "C" was not known, and the two distinct morphotypes, each with
>>relatively broad distributions, were consistently treated as distinct
>>species by all researchers over many decades.  Then, someone discovers
>>population "C".  Are you now tempted to disrupt nomenclatural stability and
>>treat them as conspecifics, or would you prefer to maintain them as
>>distinct, and note a zone of hybridization.
>
>I prefer the second alternative.  After all, dogs and coyotes are still 
>called different species.

Steve Manning